1. Technical Field
The present invention relates to a capture frequency decision methods and receivers.
2. Related Art
A GPS (global positioning system) is widely known as a positioning system using a positioning signal and is used for a GPS receiver built into a mobile phone, a car navigation apparatus, or the like. The GPS receiver performs a position calculation process for calculating the three-dimensional coordinate values, which indicates the position of the receiver, and a clock error on the basis of the information, such as the positions of a plurality of GPS satellites or a pseudo distance from each GPS satellite to the receiver.
A GPS satellite signal is a kind of communication signal spread-modulated by the CDMA (Code Division Multiple Access) method which is known as a spread spectrum modulation method. A frequency (carrier frequency) when a GPS satellite transmits a GPS satellite signal is 1.57542 [GHz], but the frequency when a GPS receiver actually receives the GPS satellite signal does not necessarily match the carrier frequency due to the Doppler effect or the like caused by movement of the GPS satellite and/or the receiver. For this reason, the GPS receiver searches for a frequency for capturing a GPS satellite signal from a received signal to decide a capture frequency.
In general, the capture frequency is decided by performing a correlation operation between a received signal and a signal of a spread code replica, which simulates a spread code of a GPS satellite signal, while changing a frequency (correlation operation in the frequency direction). For example, U. S. Patent Application Publication No. 2007/0109189 discloses a technique of searching for a capture frequency on the basis of a frequency change when a correlation operation is performed for a predetermined integration time.
The technique disclosed in U.S. Patent Application Publication No. 2007/0109189 is a technique specialized when the integration time is fixed. In practice, however, the integration time may change. For example, the integration time may be variably controlled according to the receiving environment (for example, outdoors or indoors) of a receiver or the signal strength of a received signal. A portion (peak at which a correlation operation result becomes a maximum) equivalent to a main lobe and a portion (peak at which a correlation operation result is not a maximum) equivalent to a side lobe are present in a correlation operation result. The peak of the portion equivalent to a side lobe is not a correct peak. For this reason, it leads to a reduction in the time taken to calculate the current position, and an accurate position calculation to search a correct peak by quickly extracting a portion equivalent to a main lobe from the correlation operation result.